(1) Field of the Invention
The present invention relates to a power control apparatus which is suitable for use in a battery-powered system, such as a cellular mobile telephone or a portable electronic device.
(2) Description of the Related Art
A power control apparatus provided in a battery-powered system, such as a cellular mobile telephone or a portable electronic device, is known. In the power control apparatus, a source voltage of a battery is converted into a controlled voltage, and the controlled voltage is supplied to each of driven circuits (or functional elements) of the system. There is a demand for the power control apparatus of this type to reduce a power consumption of the driven circuits. In recent years, the power control apparatus of this type has been improved for the purpose of reducing a power consumption of the driven circuits.
For example, Japanese Laid-Open Patent Application No. 5-088790 discloses a power control system which is adapted to allow a sleep-mode operation of a CPU (central processing unit) in which the operation of the CPU is assured and the power of the CPU is turned OFF.
Japanese Laid-Open Patent Application No. 5-265597 discloses a micro-controller which allows a driven circuit to operate at a low voltage in a selected mode. In the micro-controller, a source voltage supplied to the driven circuit is controlled so as to meet one of power consumption reduction, operating speed increase and noise reduction modes.
Japanese Laid-Open Patent Application No. 6-139373 discloses a semiconductor device provided with a switch selectable between a normal power mode and a power saving mode. In the semiconductor device, a voltage supplied to a driven circuit is controlled by setting the switch to select one of the two modes.
FIG. 7 shows a conventional power control apparatus. The conventional power control apparatus of FIG. 7 includes a power-supply circuit 61 which supplies a source voltage of a first battery 60a through a diode 80 to a driven circuit 71. A power-supply circuit 62 supplies the source voltage of the first battery 60a to each of a driven circuit 72 and a power-supply circuit 63. The power-supply circuit 63 supplies a lower voltage, derived from the source voltage of the first battery 60a or from the power-supply circuit 62, to a driven circuit 73. A source voltage of a second battery 60b is supplied through a diode 81 to the driven circuit 71.
In the conventional power control apparatus of FIG. 7, the second battery 60b and the diode 81 constitute a backup power supply that acts to supply the source voltage of the second battery 60b to the driven circuit 71 when the source voltage of the first battery 60a supplied to the driven circuit 71 by the power-supply circuit 61 is discontinued. The power-supply circuit 63 acts as a dependent circuit that operates in dependence on a power-supply operation of the power-supply circuit 62. That is, the dependent power-supply circuit 63 operates to supply the lower voltage to the driven circuit 73 when the power-supply circuit 62 is operating. The dependent power-supply circuit 63 may include a DC-DC converter or a voltage regulator.
The relationship between the power-supply circuit 62 and the dependent power-supply circuit 63 is needed when the conventional power control apparatus includes functional blocks indicated by a dotted line in FIG. 7. That is, the relationship between the power-supply circuit 62 and the dependent power-supply circuit 63 is needed when one of the functional blocks (for example, the driven circuit 72) is driven by the power-supply circuit 72 at the source voltage of the battery 60a while the other functional block (for example, the driven circuit 73) is driven by the power-supply circuit 63 at the lower voltage derived from the source voltage.
In the conventional power control apparatus of FIG. 7, each of the driven circuits 71, 72 and 73 is provided with an oscillation circuit. When the oscillation circuit generates a clock signal with a lowered frequency and supplies the clock signal to the driven circuit of concern, the driven circuit is set in a low-speed mode so that it operates at a low speed. When the oscillation circuit stops supplying the clock signal, the driven circuit of concern is set in a stand-by mode. By using the oscillation circuits of the driven circuits 71, 72 and 73, the conventional power control apparatus of FIG. 7 acts to reduce the power consumption of the driven circuits 71, 72 and 73.
In the conventional power control apparatus of FIG. 7, if the source voltage is continuously supplied to the driven circuit of concern during the stand-by mode, it is difficult to completely prevent the flow of a leak current from a MOS (metal oxide semiconductor) transistor of the driven circuit. In order to eliminate this problem, it is necessary for the power-supply circuits 61 and 62 to stop the supplying of the source voltage of the first battery 60a to the driven circuits 71 and 72 when the driven circuit is in the stand-by mode.
However, in the conventional power control apparatus of FIG. 7, the power-supply circuit 63 continues to supply the lower voltage to the driven circuit 73 during the operation of the power-supply circuit 62. Even when the operation of the driven circuit 73 is not needed, the power-supply circuit 63 continuously supplies the lower voltage to the driven circuit 73. In such a condition, the driven circuit 73 unnecessarily consumes the power supplied by the power-supply circuit 63, and the conventional power control apparatus of FIG. 7 does not act to reduce the power consumption of the driven circuit 73.